VINYLOGOUS BETA-CAROTENES - GENERATION, STORAGE, AND DELOCALIZATION OF CHARGE IN CAROTENOIDS

In this paper, we report on the synthesis of beta-carotenes of variabl e chain length with between 5 and 23 double bonds (1-7). These oligoen es were prepared by McMurry and Wittig reactions. The tetradecapreno-b eta-carotene 7 would seem to be the longest p-carotene yet reported. F urthermore, we investigated the electronic properties using cyclic vol tammetry and photoelectron spectroscopy (UPS) to generate open-shell a nd closed-shell ions of carotenoids in solution and in the solid state , respectively. With increasing chain length (greater than or equal to 11 double bonds), even the generation of tetracations and tetraanions could be observed by cyclic voltammetry. Extending the number of conj ugated bonds causes the potentials to converge to Limiting values. AU electron-transfer processes occur in one-electron steps, which are clo se to each other in pairs. The potential difference between the first oxidation potential and the first reduction potential is a linear func tion of the reciprocal chain length. Despite the different techniques used (CV and UPS) and the different condensed phases, there is an exce llent correspondence between the energies of the radical cation states generated by the two methods. This shows that the radical cation form ation is principally determined by the chain length. The structures of the ion states were investigated using semiempirical methods at the N DDO level. Charge delocalization and bond relaxation are not identical and do not utilize the same number or the same kind of atoms. It can be shown that from the length of 20 double bonds onwards, the effectiv e conjugation length for doubly-charged cations converges slowly to a limiting value.